An internal combustion engine in which coolant water is circulated by a way of so-called breadthwise flowing is described in Japanese Utility Model Laid-Open publication No. 64-34423. More particularly, the above publication discloses a cylinder block having three cylinders placed in series. This cylinder block is provided with two coolant water passages independent of each other on either side of the three cylinders. One of the coolant water passages is equipped with an inlet of coolant water. The other coolant water passage is equipped with an outlet of the coolant water.
A cylinder head is attached to the cylinder block. The cylinder head is provided with coolant water passages for cooling down surroundings of valve systems installed therein. Usually, this coolant water passage delivers and receives coolant water to and from coolant water passages in the cylinder block. According to the internal combustion engine disclosed by the above described publication, coolant water supplied to the inlet initially flows through inside of the coolant water passage installed in one side (it is assumed exhaust side) of the cylinder block. Subsequently, the coolant water flows into the coolant water passage in the cylinder head from openings provided at its exhaust side. The coolant water flowing through the inside of the cylinder head flows into the other side of the cylinder block, that is, into the coolant water passage installed in the intake side from intake side openings. The coolant water circulates along sides of the cylinders afterwards, so as to flow out from the outlet. As described above, the internal combustion engine disclosed by the publication can circulate the coolant water around the cylinder and the valve system by the way so-called breadthwise flowing.
As a technique to circulate coolant water inside of the internal combustion engine, the technique of so-called lengthwise flowing is known other than the technique of the breadthwise flowing. At Japanese Patent Laid-Open publication No. 2002-161743, an internal combustion engine in which coolant water is circulated by the technique of lengthwise flowing is disclosed. In this internal combustion engine, a ringed coolant water passage that is formed to surround a plurality of cylinders is provided to the cylinder block. In this case, the coolant water circulates along ringed coolant water passage so as to cool down the plurality of cylinders without distinguishing passage of the exhaust side from the passage of the intake side.
Patent document 1: Japanese Utility Model Laid-Open publication No. 64-34423
Patent document 2: Japanese Patent Laid-Open publication No. 2002-161743